Intraocular Lenses (IOLs) are frequently used for restoring or improving visual performance, such as after cataract surgery. Because an IOL may be selected from various providers and with differing IOL characteristics, reliable systems and methods to select IOLs having IOL characteristics that achieve the desired refractive outcome for a patient, such as in terms of spectacle correction and/or image quality, are needed. More particularly, it is typically desirable to select an IOL that will substantially achieve emmetropia for the patient after surgery, independent of the refractive state of the patient prior to implantation. The term emmetropia, and variations thereof, is used herein to indicate a state of vision in which an object at infinite distance from the subject eye is in sharp focus with the eye's lens in a neutral state.
The IOL characteristics necessary to achieve emmetropia are often calculated using empirical regressions. For example, the Saunders, Retzlaff, and Kraff formula (SRK) is a regression formula empirically derived from clinical data to indicate the optimal power for an IOL. The SRK regression formula is:P=A−2.5*AXL−0.9*K 
where P is the IOL power, A is the lens constant, AXL is the axial length in millimeters, and K is the average corneal curvature in diopters. Unfortunately, the SRK regression formula may yield inadequate indications, which has led to the development of the SRKII and SRK/T formulae.
More particularly, in the SRK/T method, the calculation is partially based on a previous regression analysis to predict the position of the IOL in the eye after surgery. Once the position is known, the IOL power to implant is calculated by simple paraxial optics, taking into account that the eye is a two lens system (wherein the two lenses are the cornea and the IOL), focusing on the retina. This approach is based on Fyodorov's theoretical formula.
There are numerous other formulae for calculating IOL characteristics, such as the Haigis, Hoffer Q, Olsen, and Holladay 1 and 2 models, for example. An in-depth analysis of IOL power calculation methods is provided in Shammas H J (ed.), Intraocular Lens Power Calculations, Thorofare, N.J.; Slack (2004).
Current power calculation procedures are paraxial and by definition do not account for spherical aberration present in cornea and IOL. Ray tracing procedures include wavefront aberrations but this is not a common tool in current clinical practice.
Various IOLs are designed to correct for either no corneal spherical aberration, or, at best, for the average corneal spherical aberration, present in a cataract population. Further, these IOL lenses, whether designed to correct for no corneal spherical aberration or the average corneal spherical aberration, are typically designed based solely on the average distance between the cornea and the implanted IOL. However, it is well understood that, in a typical sample of patients, the corneal spherical aberration may vary well outside the average range, as may the distance between the cornea and the IOL upon implantation. These variations may occur, for example, due to the patient's preoperative state, due to the surgical precision, or due to the healing process likely for a given eye configuration, for example. Available lenses typically do not provide post-operative spherical aberration compensation for patients having non-average eye characteristics prior to implantation. Since the wavefront aberrations change as a wavefront propagates through the eye, a procedure to predict the spherical aberration at the pupil plane creates the possibility to design and to select an IOL to obtain a desired ocular spherical aberration.
Post-lasik eyes are a particular example of eyes that are not “average”. For example, the post-lasik eye may have characteristics that are difficult to measure due to the surgical modifications to the eye, and it is well understood that these surgical modifications to the post-lasik eye, such as the decoupling that occurs between the anterior and posterior corneal radius after lasik, make certain of the eye characteristics calculated for “average” patients inaccurate for postlasik eyes. Thus, it is well known that it is exceedingly difficult to provide a recommended IOL having characteristics that will produce the desired refractive outcome and residual ocular spherical aberration for post-lasik patients.
More particularly, for example, it has been widely reported that standard lasik procedure may typically generate large amounts of corneal aberrations. This may be inferred because post-lasik patients typically present higher amounts of corneal aberrations, likely due to the lasik surgery, than would an “average” patient. Such aberrations should not be excluded in the calculation of recommended IOL characteristics if the desired refractive outcome is to be obtained.
Thus, the need exists for an apparatus, system and method for recommending an IOL having characteristics likely to provide an improved visual outcome by accounting for at least the post-operative spherical aberration at the iris plane and that is simpler than ray tracing. This need may be met, for example, by accounting for a particular patient's expected anterior chamber depth (ACD), and more particularly by accounting for a non-average distance between the cornea and the implanted IOL, and/or by additionally considering a particular patient's variation from the average corneal spherical aberration.